I am attempting to start creating a custom endpoint for one of my sites, so I can pull in ACF fields to another.  I have the following code in my theme's functions.php

    // register the endpoints needed
    add_action( 'rest_api_init', function( ) {

        /*
        // post types to include
        $_pts = array( 'posts', );
        

        // loop the post types and create a rest endpoint for the ACF fields for them
        foreach ( $_pts as $_pt ) {

            // var_dump($_pt . '/acf');

            // register the rest endpoint
            register_rest_route( 'kp-api/v1', $_pt . '/acf/(?P<id>\d+)', array(
                'methods' => array( 'GET', ),
                'get_callback' => function( $data ) {
                    return 'HERE';
                },
                'args' => array(
                    'id' => array( 'validate_callback' => function( $param, $request, $key ) {
                        return is_numeric( $param );
                    } ),
                ),
            ) );
        }
        */

        // register the rest endpoint
        register_rest_route( 'kp-api/v1', '/TEST/(?P<id>\d+)', array(
            'methods' => array( 'GET', ),
            'get_callback' => function( $data ) {
                return 'HERE';
            },
            'args' => array(
                'id' => array( 'validate_callback' => function( $param, $request, $key ) {
                    return is_numeric( $param );
                } ),
            ),
        ) );

    } );

And when I try to browse my endpoint, I end up with `{"code":"rest_no_route","message":"No route was found matching the URL and request method","data":{"status":404}}`

And, I cannot see the endpoint in the site's /wp-json/.    I can however, see that at least the namespace itself is indeed created by browsing to it: `/wp-json/kp-api/v1`

However, there are no endpoints in it.

    {
	  "namespace": "kp-api/v1",
	  "routes": {
	    "/kp-api/v1": {
	      "namespace": "kp-api/v1",
	      "methods": [
		"GET"
	      ],
	      "endpoints": [
		{
		  "methods": [
		    "GET"
		  ],
		  "args": {
		    "namespace": {
		      "required": false,
		      "default": "kp-api/v1"
		    },
		    "context": {
		      "required": false,
		      "default": "view"
		    }
		  }
		}
	      ],
	      "_links": {
		"self": "https://example.com/wp-json/kp-api/v1"
	      }
	    }
	  },
	  "_links": {
	    "up": [
	      {
		"href": "https://example.com/wp-json/"
	      }
	    ]
	  }
	}

What am I doing wrong?   No, I'm not using the wp-json plugin, just the normal built-in Wordpress functionality (v.5.3.2)